It is well known that certain ceramics have unique electrical superconducting characteristics and properties which make them extremely well suited for a wide range of specific applications. Indeed, many additional applications for these so-called ceramic superconductors are still being determined. Ceramic superconductors, however, are quite brittle and can be easily damaged. Thus, to a very great extent, the effectiveness of any particular application for the superconductor depends on the ability to establish a platform which will support the ceramic superconductor for its intended purpose.
Such a supporting platform is disclosed and claimed in the U.S. patent application of Woolf, et al., Ser. No. 07/265,827, which is assigned to the same assignee as the present invention. The particular structure disclosed is a metallic wire substrate which supports a superconductor having constituent elements that include a rare earth (RE) together with Barium (Ba), Copper (Cu), and Oxygen (O). Typically, this superconductor is designated RE Ba.sub.2 Cu.sub.3 O.sub.7-x and is familiarly known as a 1-2-3 superconductor.
Ceramic superconductors are formed in polycrystalline structures and, generally, the 1-2-3 crystals are randomly oriented in the ceramic with respect to each other. As is well known, ceramic objects are produced according to a selected process and normally the orientation of individual crystals within the ceramic is of no concern. On the other hand, insofar as superconducting ceramics are concerned the particular crystalline structure of the ceramic is important for several reasons. Firstly, on the atomic level, the structures within these crystals are highly oriented. It so happens that the particular geometric orientations of these structures define the directions along which the crystal will preferentially grow. It also happens that superconducting crystalline structures are capable of conducting more current per unit area along the preferential dimensions of growth than along the dimension of non-preferential, or slower, growth. Accordingly, a superconducting ceramic will exhibit improved superconducting characteristics if the individual 1-2-3 crystals of the ceramic are aligned in a co-parallel orientation and are elongated in the direction of current flow.
In addition to the factor discussed above, the improved superconducting characteristics associated with superconductor crystalline structures having aligned, co-parallel grains are attributable to other factors as well. For instance, the boundaries between elongated crystals, which are similarly aligned in a side-by-side relationship relative to the direction of current flow, have more intersurface contact which can carry a higher superconducting current than do crystal boundaries that are formed by crystals which do not share a common crystalline alignment. Indeed, the boundaries themselves between colinearly aligned crystals have been shown to be capable of carrying more current flow than the boundaries of unaligned crystals. Secondly, crystalline structures having aligned, co-parallel crystal grains tend to have less cracking in the overall crystal structure than structures having unaligned grains, and hence have improved superconducting characteristics. In particular, one such improved superconducting characteristic is that the critical current (J.sub.c) of a ceramic superconductor whose individual crystals are so aligned and elongated is larger and less affected by a magnetic field. Specifically, with its crystals properly aligned, J.sub.c for a ceramic superconductor with aligned crystals does not decrease as rapidly in the presence of a magnetic field as does the J.sub.c of a superconductor whose individual crystals are randomly oriented with respect to each other.
To provide for ceramic crystal alignment, several processes have been proposed which, when applied to ceramic crystalline structures, approximately orient the crystal grains in a given direction. Substantial intergranular misalignments, however, may still exist after ceramic structures have been processed by many of these proposed techniques. One generalized technique which ameliorates the misalignment problem by producing crystalline structures having substantially aligned and elongated crystal grains is the technique of heat texturing. This technique requires melting the ceramic structure and then establishing a thermal gradient across the structure as it cools. The effect of this technique is that the individual crystal grains which renucleate in the cooling melt and are aligned with their fast growing direction along the temperature gradient grow much faster than other grains and dominate the structure of the recrystallized ceramic.
Another process related to heat texturing which also results in substantially aligned, elongated crystal grains is the process known as Ostwald Ripening. This process first requires doping the ceramic crystalline structure with a liquid carrier, such as silver (Ag), cuprous oxide (CuO), barium oxide (BaO), or combinations of the above compounds and element. Then, the doped ceramic is heated to above the melting point of the carrier, but below the melting point of the ceramic material When so heated, however, it happens that some of the smaller ceramic crystal grains tend to enter into solution with the liquified carrier element. The extent of this dissolution is strongly dependent upon both temperature and crystal size. A proper temperature gradient is then established along the doped ceramic structure. The dissolved ceramic diffuses from regions of high solubility (higher temperature and smaller crystals) to those of low solubility (lower temperature and larger crystals), dissolving material from the high temperature end of the gradient and from small crystals, and redepositing it at the low temperature end of the gradient and preferentially onto larger crystals. Accordingly, larger crystals oriented along the temperature gradient can benefit more from this deposition because they have a larger surface area for redeposition, and can grow so that their rapid growth faces stay in the growth region longer. On the other hand, smaller, incorrectly oriented crystals tend to be dissolved. Thus, the resulting microstructure is elongated in the direction of the temperature gradient.
The particular heat texturing related techniques proposed in the past, however, are suitable only for applications involving short, relatively large cross-section ceramic bars. They are generally unsuitable for treating ceramic crystals in very long superconducting wire that are fine enough to be flexible.
It would therefore be an advancement in the art to provide a substantially long, flexible superconducting wire whose individual superconducting ceramic crystals are substantially aligned and elongated in the direction of current flow. It would be another advancement in the art to provide a superconducting ceramic with the above crystalline properties when the ceramic is bonded to a flexible wire-like substrate base. Still another advancement in the art would be to provide an electrically superconducting wire of indeterminate length, the superconducting characteristics of which are still effective in the presence of a magnetic field. It would also be an advancement in the art to produce an electrically superconducting wire at rates sufficient to make, in economic time spans, lengths of wire that are up to several kilometers long. Yet another advancement in the art would be to provide a ceramic superconductor which is relatively easy to manufacture and comparatively cost effective.